Just installed Walmart's EverStart H8 AGM battery ($149) 900 CCA and it fit perfectly.
https://www.walmart.com/ip/EverStart...up-h8/40647529. It has 2 top vent holes and comes with one vent hole plug (it's on the red plastic positive pole cap that should come with the battery), so gotta plug the other hole on the negative side.
And boy oh boy, my X5 has woken up and is alive as ever! It's not in my head but she's more responsive and definitely has that extra oomph. When I hit the pedal to the metal, she takes off immediately - no that milliseconds of delay I used to have. The vehicle just generally feels alive and nimble. Starts immediately as well. Wow, didn't know what I was missing.